1. Field of the Invention
The present invention relates to a method of doping a semiconductor with an impurity. In particular, the present invention relates to channel doping, where precise control of the added amount of an impurity with which a semiconductor is to be doped is necessary, and to a laser doping method where an impurity is added to a semiconductor by irradiating the semiconductor with laser light.
2. Description of the Related Art
When a semiconductor device is fabricated using the electrical characteristics of a semiconductor, a technique called doping, where a minute amount of an impurity is added to a semiconductor in order to control the electrical characteristics thereof, is at present widely used. Methods of doping semiconductors can be broadly divided into two kinds: methods where the impurity is added in the process of forming the semiconductor from raw materials, and methods where the impurity is added after the semiconductor has been formed. Examples of methods where the impurity is added after the semiconductor has been formed representatively include ion implantation, plasma doping and laser doping.
Ion implantation is a method where a gas including a dopant is added to a plasma chamber to draw out ion beams, and desired dopant ions are accelerated in a mass separator with an energy of several keV to several hundred keV and caused to impact the surface of the semiconductor to physically add the dopant. Plasma doping is a method where doping is conducted by exposing the semiconductor to plasma included a dopant gas diluted to a desired concentration or by exposing the semiconductor to ions drawn out from the plasma. Because mass separation is not conducted in plasma doping, the semiconductor is doped with various forms of ions generated in the plasma.
Laser doping is also used as one form of doping. Laser doping is a method where the semiconductor is disposed in a gas including a dopant and the surface of the semiconductor is irradiated with laser light, whereby the semiconductor is melted, and the dopant is added and activated. Although wide-range doping and high activation are possible with laser doping, in order to further raise the efficiency of laser doping, research is also being conducted with respect to heating samples or irradiating the samples with a laser while imparting magnetic energy thereto (e.g., see Patent Document 1).
In the world of semiconductors, advances with respect to increasing speed and miniaturization are being made at a fast pace. In accompaniment therewith, there is a strong demand to improve the semiconductors themselves and to improve circuit reliability. Although the threshold voltage is often used as a parameter to evaluate semiconductor devices, the threshold voltage lacks reliability if it is too high or too low, and the most important thing is being able to obtain a desired threshold voltage.
A technique called channel doping is used as a means for controlling the threshold voltage. As the name implies, channel doping is a technique where an infinitesimal amount of an impurity is added to a portion serving as a channel forming region so that a desired threshold voltage can be obtained. Because the purpose of channel doping is to precisely control changes in the threshold voltage, it is necessary to precisely control the added amount of the impurity.
The amount of ions implanted in ion implantation or plasma doping is controlled using a Faraday Cup Electrometer, which measures flowing ions as a current. Ion implantation, which can dope the semiconductor with just the necessary type of ions, can be suitably used in channel doping where precise concentration control is necessary.
In order to realize an increase in speed and miniaturization, it is also necessary to shorten the gate length. However, this is a problem because, when the gate length is shortened, the so-called short channel effect arises where the current leaks at the deep portion of the channel forming region. In order to prevent this, it is effective to add the impurity only to extremely shallow portions of the source region and the drain region (to form shallow junctions).
As a technique for forming a crystalline semiconductor layer with respect to a substrate with low thermal resistance, such as a glass substrate, there is a technique that uses elements represented by Ni which promote crystallization. This is a technique where crystallization is conducted at a low temperature and in a short amount of time by forming amorphous silicon on a glass substrate, forming a thin film of Ni by sputtering or applying a solution including Ni with a spinner, and conducting crystallization. By using an inexpensive glass substrate, this technique is important in lowering the cost of the product.
[Patent Document 1]
JP-A-5-326430
The present state of affairs is one where there are problems with doping in that the sizes of substrates flowing through the fabrication process are becoming larger and the regions where doping is necessary are expanding in accompaniment with circuits that are becoming more integrated and displays that are becoming larger year after year.
In ion implantation, in order to accurately mass-separate the ion beams, it is necessary to narrow the beams. For this reason, there are grave problems in that ion implantation cannot accommodate wide-range doping because doping cannot be conducted at one time over a wide range, and an enormous amount of time is required for processing in order to dope a large area.
With ion implantation, there is also the problem that it is necessary to implant ions with a low energy in order to form shallow junctions, but low-energy ion beams end up being emitted and it is difficult to obtain a sufficient current amount. It is also extremely difficult to add the impurity to only the extremely shallow regions because the dopant ends up being diffused due to thermal annealing for recovering damage resulting from the ion implantation.
Plasma doping is more effective than ion implantation in terms of processing time, but because mass separation is not conducted, the ion current measured by a Faraday Cup Electrometer is the total ion current where the ions of the gas diluting the dopant gas are added to the dopant ions. Thus, even if the total ion dopant amount is precisely controlled, the amount of the implanted dopant also ends up changing when the ratio of the gas diluting the dopant gas to the dopant gas, i.e., the ion ratio, changes during device operation. For this reason, there is a problem in applying plasma doping to channel doping, where precise concentration control is necessary.
With respect to laser doping, in methods that are often conducted where the semiconductor is irradiated with laser light in a dopant gas atmosphere, controlling the amount of the dopant to be implanted is difficult and a special device is required in order to uniformly distribute the dopant.
An other problem is that, although elements that promote crystallization have an extremely effective function in relation to crystallization, when these elements remain in large amounts in the semiconductor layer after crystallization, they have an adverse affect on TFT characteristics when a TFT—and particularly a TFT channel region—is made using the semiconductor layer. For this reason, when crystallization is conducted using elements that promote crystallization, it is common for means called gettering, which reduces the concentration of such elements, to be taken.
Gettering is conducted by moving, to another region (called a gettering region), the elements that promote crystallization from regions where it is a problem for those elements to remain.
In relation to removing the remaining element Ni, a method according to gettering (JP-A-10-214786) has been disclosed by the present applicant; however, a mask forming step for selectively adding the Ni element and a mask forming step for selectively adding a gettering element are necessary, and there is the problem of increasing steps, such as there being the necessity of having to twice conduct heating in the crystallization step and the gettering step, which causes productivity and costs to deteriorate.